The present invention relates to a novel synthesis of octreotide and derivatives thereof by solid phase peptide synthesis. In particular, the present invention relates to synthesize linear peptides, cleave from resin and the cyclization process for their preparation.
Octreotide is a somatostatin analog which is inhibited the growth of tumor cell by binding the analog to the somatostatin receptor that located on the surface of tumor cell. The high binding affinity of the octreotide with the somatostatin receptor has been utilized as a tumor-visualization agent by labeled the octreotide with radioisotope. In-DTPA-octreotide (Octreo-Scan III) have been successfully used to visualized somatostatin receptor-positive tumors by gamma camera scintigraphy.
Octreotide comprises 8 amino acids which has the following structural formula: 
wherein sulfur atoms of the Cys at the position 2 and of the Cys at the position 7 are mono-cyclic to form an -S-S-bridge. Tyr-3-octreotide is a octreotide derivative thereof which Phe at the position 3 is replaced by Tyr.
Synthesis of octreotide and derivative thereof can be carried out by two methods. The first method is synthesized initially by fragment condensation solution phase procedures. The synthetic process of octreotide has been described by Bauer et al (Eur. Pat. Appl. 29,579,1981 and U.S. Pat. No. 4,395,403, 1983).
The process comprises:  less than i greater than  removing protected group from peptide;  less than ii greater than  linking together by an amide bond two peptide unit;  less than iii greater than  converting a function group at the N- or C-terminal;  less than iv greater than  oxidizing a straight chain polypeptide by boron tristrifluoroacetate. This process involves a time-consuming, multi-step synthesis, and it is difficult to separate octreotide from the reaction mixtures since all the synthesis steps are carried out in liquid phase.
The second method is synthesized by solid-phase procedures. Edward et al isolated side chain protected [D-Trp(Boc)4, Lys(Boc)5, Thr(But)6]-octreotide with a total yield of 14% by cleaving the protected peptide from the resin with threoninol (J. Med. Chem. 1994,37,3749-3757). Arano et al carried out another solid phase method for DTPA-octreotide (Bioconjugate Chem. 1997,8,442-446). Iodine oxidation of the DTPA-peptide produced DTPA-D-PheI-octreotide in overall 31.8% yield based on the starting Fmoc-Thr(tBu)-ol-resin.
Wu et al developed a method to synthesis of octreotide (Tetrahedron Letters 1998,39,1783-1784). The formation of the disulfide bond was oxidized using a dilution solution of octreotide with air and spent 48 hours. Lee et al recently have carried out a new method to anchor Thr(ol) to a solid phase synthesis resin for preparation of octreotide (U.S. Pat. No. 5,889,146, 1999). Fmoc-Thr(ol)-terephthal-acetal was loaded onto the resin. After construction of peptide chains by Fmoc chemistry, cyclization of the peptide may be obtained on resin by oxidation with iodine. The cleavage of peptide-resin with trifluoroacetic acid, produced octreotide with overall yield of  greater than 70% from the starting Fmoc-Thr(ol)-tere-phthal-acetal-resin. All of these procedures completed the cyclization of the octreotide either on totally deprotected peptide or on the resin.
The main object of the present invention is to provide a novel process for synthesis of octreotide and derivatives thereof. The starting material, Thr(ol)(tBu)-2-chlorotrityl resin is coupled with the various amino acids. The straight peptide-resin, such as
D-Phe-Cys(Trt)-Phe-D-Trp(Boc)-Lys(Boc)-Thr(tBu)-Cys(Trt)-Thr(ol)(tBu)-2-chlorotrityl resin or
D-Phe-Cys(Trt)-Tyr(tBu)-D-Trp(Boc)-Lys(Boc)-Thr(tBu)-Cys(Trt)-Thr(ol)(tBu)-2-chlorotrityl resin was obtained. Cleavage of the peptide from the resin was achieved by strong acid solution.
The cleaved peptide reacted with charcoal to give disulfide-containing peptide as following: 
Purification of the crude peptide was obtained by chromatography. The novel process in this invention for synthesis of octreotide and derivative thereof has been proved to be more time-saving, higher yield  greater than 80%, and easier for separation from the reaction system than the prior art is.
The following abbreviations are employed:
Fmoc: 9-fluorenylmethoxycarbonyl
Boc: t-butyloxycarbonyl
tBu: tert-butyl
Trt: triphenylmethyl
Thr(ol): the threoninol residue
Phe: the phenylalanine residue
Cys: the cysteine residue
Thr: the threonine residue
Lys: the lysine residue
Trp: the tryptophan residue
Tyr: the tyrosine residue
TFA: trifluoroacetic acid
EDT: 1,2-ethanedithiol
THF: tetrahydrofuran
HBTB: Benzotriazolyloxytetramethyluronium hexafluorophosphate